(1) Field of the Invention
The present invention relates to a stepping motor control circuit, and more particularly to a stepping motor control circuit which can set a two-phase excitation system and one-two phase excitation system or two-one phase excitation system, and also which can provide the stepping motor with the same operating timing in its both normal and reverse operation in the state in which the two-phase excitation system is set.
(2) Description of the Related Art
Referring to FIGS. 1 to 3A and 3B, an explanation will be given on the conventional stepping motor control circuit.
FIG. 1 is a block diagram showing an arrangement of the conventional stepping motor control circuit.
As seen from FIG. 1, a stepping motor control circuit 50 for driving a head included in a floppy disk drive (FDD) or a hard disk drive (HDD) is composed of a step circuit 51 and a track circuit 52 for converting the stepping pulse signal sent from a host device (computer) 49 into a suitable signal. The excitation outputs thus obtained by the stepping motor control circuit 50 is used to control a stepping motor 55. The step circuit 51 in the control circuit 50 converts a stepping pulse signal X from the host computer 49 into a pulse train Y of a plurality of pulses which are to be sent to the following track circuit 52. The track circuit 52 is composed of an up-down counter circuit 1A for counting the pulse train and a decoder circuit 2 for decoding the counter output. The stepping motor 55 is excited on the basis of the decoded output Z.
A number of excitation methods for such a stepping motor have been proposed. Here, with respect to a four-phase stepping motor, as an example, an explanation will be given on the track circuit 52 capable of setting a two-phase excitation system and a one-two phase excitation system or two-one phase excitation system.
FIG. 2 is a detailed circuit diagram of the conventional track circuit 52 of FIG. 1. As seen from FIG. 2, the conventional track circuit 52 comprises the up-down counter circuit 1A and the decoder circuit 2. Specifically, the up-down counter 1A is composed of two-input AND gates 22, 23 and 27, 28 which receive the signal from a counting direction setting terminal 3 directly or via an inverter 21; two-input NOR gates 24 and 29 for receiving outputs therefrom; a two-input NOR gate 25 and a wiring 12 connected with the output sides of these two-input NOR gates 24 and 29, respectively; exclusive-OR (EX-OR) gates 26 and 30; two D-type flip/flops (hereinafter referred to as "D-F/F"s) 35 and 36; and a T-type flip/flop (hereinafter referred to as "T-F/F") 37. On the other hand, the decoder circuit 2 is composed of an EX-OR gate 38, an inverter 39, a NAND gate 40, three-input NOR gates 41.about.44 and two-input NOR gates 45.about.48. The counting direction setting terminal 3 which is one of the input terminals of the up-down counter circuit 1A in the conventional track circuit 52 serves to set the direction for counting the counter 1A. The up-down counter 1A is placed in an up-counting mode with a low level (hereinafter referred to as "L") input, whereas it is placed in a down-counting mode with a high level (hereinafter referred to as "H") input. A clock pulse forwarded from the step circuit 51 is inputted to a clock terminal 4. The counter circuit 1A executes the counting operation on the basis of the clock pulse. A resetting input terminal 5 which is a reset input terminal of the counter circuit 1A serves to set an initial condition. Another input terminal, that is, an excitation system setting terminal 6 serves to set the excitation system of the stepping motor. An "H" input sets the stepping motor the one-two phase excitation system, whereas an "L" input set the two-phase excitation system. On the other hand, output terminals 7.about.10 are four phase outputs (hereinafter referred to as "excitation output"s) for exciting the stepping motor 55. These excitation output signals can be obtained by decoding the signals from the counter circuit 1A. The output terminals 7, 8, 9 and 10 output the first-, the second-, the third- and the fourth-phase output in their order.
Now, referring to FIGS. 3A and 3B, an explanation will be given on the relation between the inputs and outputs in the conventional track circuit 52, i.e., the relationship between the inputs to the counting direction setting terminal 3, the clock terminal 4, the resetting terminal 5 and the excitation system setting terminal 6 and the outputs from the NOR gate 29 and the excitation outputs.
FIG. 3A is a timing chart when the one-two phase excitation system is set in the circuit of FIG. 2. As seen from FIG. 3A, if the excitation system setting terminal 6 is set for "H" to set the one-two phase excitation system for the stepping motor, and the counting direction setting terminal 3 is set for "L" to place the counter circuit 1A in the up-counting mode, the first-phase output is "H", the second- to fourth-outputs are all "L" in a reset state. Subsequently, when the first clock is inputted, the second-phase output changes into "H", so that the excitation outputs will start to change at the timing of the first clock input. Further, in response to each input clock, the state of the excitation outputs will change in sequence. If, after the eighth clock has been inputted, the counting direction setting terminal 3 is set for "H" to place the counter circuit in the down-counting mode and the first clock is inputted thereafter, the fourth-phase output becomes "H" and thus the excitation outputs start to change from the timing of the first clock input, so that the state of the excitation outputs will change in the direction reverse to that in the up-counting mode.
Next, FIG. 3B is a timing chart when the two-phase excitation system is set in the circuit of FIG. 2. As seen from FIG. 3B, if the excitation system setting terminal 6 is set for "L" to set the two-phase excitation system for the stepping motor 55, and the counting direction setting terminal 3 is set for "L" to place the counter circuit 1A in the up-counting mode, the first- and fourth-phase outputs are "H" and the second- and third-outputs are "L" in the reset-state. Subsequently, when the first clock is inputted, the second-phase output changes into "H" and the fourth-output is changed into "L", so that the excitation outputs will start to change at the timing of the first clock input. Further, in response to every other input clock, the state of the excitation outputs will change in sequence.
Further, after the eighth clock has been inputted, if the counting direction setting terminal 3 is set for "H" to place the counter circuit 1A in the down-counting mode, the first clock inputted thereafter does not change the excitation outputs, but in response to the second clock input, the first-phase output becomes "H" and the third-phase output becomes "H" and the excitation outputs thus start to change from the timing of the second clock input. Further, in response to every other input clock, the state of the excitation outputs will change in the direction reverse to the case of the up-counting mode.
As understood from the above explanation, the conventional stepping motor control circuit has the following problems. If the two-phase excitation system is selected, the track circuit starts the output changing operation at the timing of the second clock pulse input in either one of the up-counting operation or the down-counting operation. Thus, in comparison to the other of the counting operations, there occurs a time lag in the excitation outputs corresponding to one clock interval, i.e., a half step interval. This reduces the operation speed of the track circuit. The problem may not be limited only to the reduction of the operation speed since, in a case where information on completion of the track operation is to be sent back to the host computer which provides the stepping pulse to the stepping motor control circuit, there will be a delay in sending the information thereby causing further problems.